tiltfile: add a stringable type to make it easier to unpack strings

[nicks]

Hello @maiamcc, @landism,

Please review the following commits I made in branch nicks/stringable:

0422a04 (2020-08-12 19:35:49 -0400)
tiltfile: add a stringable type to make it easier to unpack strings

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[landism]

LGTM, definite improvement over status quo.

This highlights to me that we don't have much consistency about whether a function's string parameter has to be an actual string or simply a stringable. Think it'd make sense to have our unpacker swap out all strings with stringables before calling the starlark unpacker?

[maiamcc]

ooh nice

[nicks]

@landism ya, possibly! one thing I went back and forth on is if we should have a string Unpacker that trims the whitespace/newlines off blobs, or matches them against some validity regexp, or if that would be confusing